Chromite-PGM Mineralization in the Lherzolite Mantle Tectonite of the Kraka Ophiolite Complex (Southern Urals, Russia)

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (<100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

Provenance of the Sawa Formation Sandstones, Vindhyan Super Group, Southeast Rajasthan, India

Integrated petrographical and geochemical analysis of Sawa Formation sandstones was analyzed to reconstruct their source area weathering, paleoclimate, tectonic setting and provenance conditions. Petrographically, quartz is dominant detrital mineral followed by feldspar, mica, rock fragments and heavy minerals. Sawa Formation sandstones have been classified as quartzarenite with subordinate sub-arkose and sub-litharenite type. Major oxide element abundances revealed the sandstones have high SiO2 concentration, high K2O/ Na2O ratio, which is consistent with the petrographic data. These sandstones were derived mainly from stable cratonic with minor collision suture and fold thrust belt source and deposited in rifted continental margin basin setting, reflecting high maturity of sediments and high stability of the source area. The CIA, CIW and PIA values of these sandstones indicate high intensity of weathering condition in the source area under warm and humid climate.

Trøndelag Platform & Halten-Dønna terraces Composite Tectono-Sedimentary Element, Norwegian Rifted Margin, Norwegian Sea

The Trøndelag Platform and Halten-Dønna Terraces occupy a central part of the Norwegian Sea Rifted Continental Margin off the mid-Norway coast. The margin is expected to hold 4500 Mbboe of undiscovered oil equivalents and represents an economically important Arctic province. The geological evolution of the area is closely linked to processes involving the Caledonian orogeny and subsequent plate tectonic re-organizations, multiphase rifting, continental drift and glaciations across the northern hemisphere. In this chapter we review the geology of the Trøndelag Platform and Halten-Dønna terraces Composite Tectono-Sedimentary Element based on published data and results of Equinor ASA in-house studies. Three new structural elements are defined for the first time: the Leka fault complex, the Vikna high and the Sula basin.

Inherited lithospheric structures control arc-continent collisional heterogeneity

From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ~4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.

Gigantic-thick turbidite succession in a Miocene fault-bounded trough of the South China Sea rifted continental margin: Examples from Taiwan

&lt;p&gt;The Loshui Sandstone, a Miocene turbidite succession accumulated in the northern slope of the rifted continental margin of the South China Sea, is exposed in the Hengchun Peninsula, Taiwan. We conduct lithofacies analysis to understand the depositional processes and mechanisms of the gigantic-thick turbidite succession.&lt;/p&gt;&lt;p&gt;Several features can be recognized from outcrops: (1) the Loshui Sandstone is of around 1,000 m thick with turbidite units stacked vertically; (2) high net-to-gross ratio (&gt; 0.9) with dominant fine-to-medium grained sandstones, amalgamated beds are commonly found in the very thickly-bedded turbidites; (3) thick individual turbidite beds with a nominal thickness of 70 cm, which is thicker than classical Bouma sequence; and (4) limited deep scouring surfaces and thick mud are found. Two end-member lithofacies of high-density turbidites and low-density turbidites, respectively, are identified. High-density turbidites are thicker (more than 1 m thick) and coarser in grain size (mostly medium sands) with abundant massive intervals, dewatering structures and/or climbing ripples. Low-density turbidites tend to be thinner in thickness and finer in grain size (mostly fine sands) with parallel bedding and/or normal ripples. In addition to the above two lithofacies, chaotic deposits of mass transport deposits (MTDs) are also widespread within the studied succession.&lt;/p&gt;&lt;p&gt;Sand-rich, vertically aggrading succession, but lack of deep-scouring surfaces and levee deposits, indicates that turbidites are laid down by unconfined turbidity currents in a sand-rich deepwater lobe. In addition, gigantic thick turbidite unit stacked continuously up to 1,000 m, implying that the lobe is confined within a rapidly subsiding basin. We interpreted that the Loshui Sandstone is vertically stacked and accumulated within a fault-bounded trough in the deepwater area of the rifted continental margin of the South China Sea.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Quantitative analysis for compaction trend and basin reconstruction of the Perth Basin, Australia: Limitations, uncertainties and requirements

&lt;p&gt;This study quantifies compaction trends of Jurassic-Quaternary sedimentary units in the Perth Basin, and applies the trends to reconstruct the sedimentation and subsidence history with 2D and 3D models. BasinVis 2.0, a MATLAB-based program, as well as MATLAB 3D surface plotting functions, &amp;#8216;Symbolic Math&amp;#8217; and &amp;#8216;Curve Fitting&amp;#8217; toolboxes are used to analyze well data. The data were collected from fourteen industry wells and IODP Site U1459 in a study area (200x70 km2) on an offshore part of the basin, which were arranged for four successive stratigraphic units; Cattamarra, Cadda, Yarragadee, and post-breakup sequences. The Perth Basin is a large north-south elongated sedimentary basin extending offshore and onshore along the rifted continental margin of southwestern Australia. It is a relatively under-explored region, despite being an established hydrocarbon producing basin. The basin has developed by multiple episodes of rifting, drifting and breakup of Greater Indian, Australian and Antarctic plates since the Permian. The basin consists of faulted structures, which are filled by Late Paleozoic to Cenozoic sedimentary rocks and sediments. After deltaic-fluvial and shallow marine deposition until early Cretaceous time, carbonate sedimentation has prevailed in the basin, which is related to the post-rift subsidence and the long-term northward drift of the Australian plate.&lt;/p&gt;&lt;p&gt;High-resolution porosity data of Site U1459 and well Houtman-1 were examined to estimate best fitting compaction trends with linear, single- and two-term exponential equations. In the compaction trend plot of Site U1459 (post-breakup Cenozoic carbonates), the linear and single-term exponential trends are relatively alike, while the two-term exponential trend has abrupt change near seafloor due to highly varying porosity. The compaction trends at well Houtman-1 (Jurassic sandstones) are alike in the estimated interval, however initial porosities are quite low and different. In the compilation plot of the two wells, the two-term exponential trend presents better the porosity distribution, by adopting a trend change as estimation overfitting, by the lithologic transition from carbonates to sandstones. The abrupt trend change suggests that the multiple piece-wise compaction trend is suitable for the Perth Basin. The compaction trends are used to quantify the sedimentation profile and subsidence curves at Site U1459. 2D and 3D models of unit thickness, sedimentation rate and subsidence of the study area are reconstructed by applying the exponential trend to the stratigraphic data of industry wells. The models are visualized using the Ordinary Kriging spatial interpolation. The results allow us to compare differences between compacted (present) and decompacted (original) units through depth and age. The compaction trend has an impact on thickness restoration as well as subsidence analysis. The differences become larger with increasing depth due to the rising compaction effect during burial. Other factors can deviate the compaction trend further through age. This phenomenon highlights the fact that the restoration of largely compacted (usually deeper or older) layers is crucial to reconstruct sedimentation systems and basin evolution. This has often been underestimated in academic and industry fields. This study suggests that researchers apply the appropriate compaction trend estimated from on-site data for basin reconstruction and modelling.&lt;/p&gt;